Just remember, the 'arduino', in terms of hardware is nothing new, I, like many, were using the Atmega88 in almost all of my projects before it arrived. Its just a convenient form factor, with a USB-serial bridge, and a regulator, most of the 'arduino' is the software used. You won't need the USB-serial bridge (but handy to have one for flashing with a bootloader) and your regulator should probably be something a bit more 'bullet-proof' to handle plugging into the automotive supply (which has load dumps and 100v spikes..). But the regulator would be needed for *any* digital circuit. A ULN2003 as a current driver for the LED's, and you're all set.
I'll admit, I wouldn't be happy having an arduino in a semi-critical project, but mostly because of the software used, and the regulator, but AVR, PIC, MSP, the micro choice is down to you,
Not to mention the almost endless possibilities of patterns, PWM fading, and hooking it up to the cars CANbus (more hassle than its worth I think).